In a hybrid structure of a construction machine such as a power shovel, power is generated by rotating a power generator using a surplus output of an engine, the power is stored in a battery, and an electric motor is driven using the power of the battery so as to activate an actuator, for example. Power is also generated by rotating the power generator using energy discharged from the actuator. This power is likewise stored in the battery, whereupon the electric motor is driven using the power of the battery so as to activate the actuator (see JP2002-275945A).
Further, in a power shovel or the like, the engine is maintained in a rotating condition even when the actuator is stopped. At such times, a pump rotates together with the engine, and therefore the pump discharges a so-called standby flow.